Chrome 6 reduction

[MMMX]

Hello, I’m emailing from Portugal, and I wonder if somebody cold give me some tips about 6+ Chrome treatments.

To make the reduction from Cr6 to Cr3 is there any alternative to sodium bisulphite?

To make the environment acid for the reduction, what is the best Chloride or Sulphuric acid?

To flocculate the remaining Cr3, what’s the best flocculation agent, and at what speed should I make the agitator work in order to prevent the floccules from breaking?

At what value of pH should I make the flocculation?

To rise up again the pH, what should I use to minimise the final sludge amount?

Thank you?

If you need some powder coat help please be free to ask.

 

[DocMax]

Hello MMMX

As you know the most frequently used technique for chrome removal is a two stage chemical treatment process. The first stage lowers the pH and adds reducing agents to convert the chrome from soluble Cr+6 to Cr+3.
The second stage neutralizes the wastewater, forming Cr(OH)3 which can be settled and removed.

First Stage

Sulfuric acid is used to lowe the pH to approx. pH 2.5-3 to promote the reduction reaction. The reducting agent my be SO2, sodium solfite, sodium bisulfite, sodium metabisulfite, sodium hydrosulfide and ferrous sulfate.
SO2 as reducting agent is restricted for large treatments plants,and sodium bisulfite is used for smaller and medium plants.
SO2 has lower chemical costs but require more expensive chemical feed systems, (sulfonator) and is a health hazard (So2 is a toxic gas).
Ferrous sulfate produce an increase in sludge generation in the neutralization phase, ferric hydroxide is produce.

Second Stage.
The wastewater is neutralized to precipitate Cr3+, an insoluble chromium hydroxide Cr(OH)3 is produce by the addition of sodium hydroxide to rise the pH to 7.5 to 8.5.
The reaction is fast, retention time of a least 10 minutes, and a pH controll will be provided.
Flocculation agents (polyelectrolites) will be fedd in order to increse the settling.

For the intensity of mixing in the flocculation stage you have to considre the typhical G value for flocculation this value is calculated with the following formula:

G = SQRT(Power/dynamic viscosity*Volume).


Ciao
Doc Max